1. Field of the Invention
The present invention generally relates to an ultrasonic motor. More particularly, the present invention pertains to a driving circuit for an ultrasonic motor to control the voltage that drives the ultrasonic motor.
2. Description of the Related Art
A conventional ultrasonic motor includes a stator on which multiple piezoelectric elements are provided, and a rotatably supported rotor that is pressed against the stator. Each piezoelectric element of the motor is separated into two portions to which high frequency voltages of different phases are applied. In accordance with the applied high frequency voltages, a progressive wave is generated on the surface of the stator, which serves to rotate the rotor.
This type of ultrasonic motor also has a driving circuit to control the high frequency voltages for driving the rotor, as seen, for example, in FIG. 7. In this driving circuit, a signal is generated at a monitor electrode 22 provided at part of each piezoelectric element 21, and is sent through an automatic tracking circuit (ATC) 23 to an oscillator 24. This oscillator 24 generates an oscillation signal in accordance with the received signal. The oscillation signal from the oscillator 24 is divided into two signals having different phases. These signals are independently amplified by switching circuits 26a and 26b, transformers 27a and 27b and the like. High frequency voltages whose phases are different from each other are applied to the respective portions of the piezoelectric element 21.
Since the signals are amplified independently in the above-described driving circuit, however, two sets of switching circuits as well as two transformers are required. The driving circuit is therefore undesirably large. Further, to drive multiple motors, multiple driving circuits have to be provided for each motor, with two transformers being provided for each driving circuit. This makes it difficult to provide a compact circuit to drive a plurality of motors.